1. Field of the Invention
The present invention relates to a process for twin sheet forming high heat distortion temperature thermoplastics and structures made therefrom.
2. Description of Related Art
Twin sheet forming processes are known in the art. (See, for example, Dresen, et. al., U.S. Pat. No. 4,428,306) Twin sheet forming processes are useful in producing rigid structures having hollow portions for reduced weight and increased strength. Prior twin sheet forming processes have employed low heat distortion temperature thermoplastics which, while being suitable for the manufacture of many items, are unsuitable for applications such as aircraft ducting which require the use of thermoplastics having good flame resistance, many of which have high heat distortion temperatures. While these known processes for twin sheet forming are suitable for producing articles from low melt temperature thermoplastics, these prior processes are typically not suitable for single layer high heat distortion temperature thermoplastics because the molds used therein if heated to a temperature high enough to achieve bonding between the sheets are too hot to permit the sheets to solidify into the desired shapes. While specialized molds employing high temperature adhesion zones and low temperature forming zones can be employed, such specialized molds add unnecessarily to the equipment costs, and the adhesion zones, usually the clamped mold peripheries, have had to be maintained at temperatures well above the heat distortion temperature of the sheets.
Accordingly, one object of the present invention is to provide a process for producing twin sheet formed structures from high heat distortion thermoplastic materials wherein the mold temperatures are below the heat distortion temperature of the high heat distortion temperature materials. Another object is to provide structures from twin sheet formed thermoplastic materials having high heat distortion temperatures.